Metamaterial reconfigurable antennas

ABSTRACT

Leaky wave antennas that can be reconfigured in pattern and/or polarization by exploiting the characteristic of metamaterial structures loaded with variable capacitor and inductors employ a Composite Right Left Handed (CRLH) unit cell with two independent DC biases used to actively change the group delay of the transmission line and the polarization of the radiated field while preserving good impedance matching. Different degrees of pattern and polarization reconfigurability are achieved by cascading multiple of these unit cells along a straight line, a circular line or a zigzag line while preserving high gain for all the antenna configurations and good impedance matching.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application No.61/286,786 filed Dec. 16, 2009.

TECHNICAL FIELD

The present invention relates generally to the field of reconfigurableantennas. Specifically, the present invention relates to antennas thatcan be reconfigured in pattern and/or polarization by using metamaterialstructures loaded with variable capacitor and inductors

BACKGROUND OF THE INVENTION

The changing behavior of the wireless channel causes fluctuations in thelevel of received signal power. In order to limit the effect of thevarying wireless channel on system performance, a possible solution isto adopt reconfigurable antenna systems capable of adaptively tuningtheir radiation characteristics in response to the multivariate channel.Radiation pattern shape, polarization state and frequency of operationcan be tuned to accommodate the operating requirements. Differentsolutions employing different techniques for reconfiguring the radiationcharacteristic have been proposed in the prior art.

Most of the proposed reconfigurable antennas achieve pattern andpolarization reconfigurability by changing the current distribution onthe antenna by means of RF switches embedded on the antennas, materialchanges or structural variations. Using these techniques allowsgenerating different polarizations and radiation patterns but,especially when the antenna has several different configurations, itgenerally causes some of the antenna configurations to suffer of lowgain or impedance mismatch. It is desired to overcome these issues andto achieve high gain pattern and polarization reconfigurable antennasthat exhibit good impedance matching for all configurations. The presentinvention has been designed to address these and other needs in the art.

SUMMARY

To address the above-mentioned needs in the art, the invention describedherein uses leaky wave antennas (LWAs) built using metamaterialstructures loaded with tunable capacitors and inductors and specific DCbias networks to control the values of capacitance and inductance acrossthe antenna. Three different reconfigurable antenna designs built usinga LWA metamaterial structure are described. These antennas exploit thecharacteristics of Composite Right Left Handed (CRLH) materials toachieve high pattern and polarization reconfigurability, with goodimpedance matching in a compact antenna design.

In particular, the invention includes metamaterial reconfigurableantennas that uses varactor diodes to change characteristics of unitcell structures such as group delay of transmission lines, polarizationand impedance, by changing the values of variable capacitors and/orinductors in response to independent DC biases provided by independentDC bias circuits. As they operate on a traveling wave and not aresonating wave basis, these antennas may enable significantimprovements in gain and reconfigurability. By controlling the varactordiodes independently, the group delay, polarization and impedance may bemore widely varied than standard unit cell structures that only changethe group delay.

In exemplary embodiments, the invention comprises a pattern and/orpolarization reconfigurable antenna comprising at least one CompositeRight Left Handed (CRLH) unit cell including a standard transmissionline with added series capacitance and shunt inductance and adapted toradiate an electrical field and at least a variable capacitance and/orinductance in series with the shunt inductance and at least a variablecapacitance and/or inductance in parallel with the series capacitance,whereby the variable capacitance and/or inductance in series with theshunt inductance and the variable capacitance and/or inductance inparallel with the series capacitance are responsive to at least two DCbiases used to independently control the variable capacitance and/orinductance in parallel with the series capacitance and the variablecapacitance and/or inductance in series with the shunt inductance tothereby control the group delay of the transmission line and apolarization of the radiated electrical field. In an exemplaryembodiment, the CRLH unit cell and the variable capacitance and/orinductance in series with the shunt inductance and the variablecapacitance and/or inductance in parallel with the series capacitanceare fabricated on a microwave laminate printed circuit board.

In different configurations of the antenna of the invention, multipleCRLH unit cells are cascaded to define a leaky wave structure that hasat least two input ports for accepting excitation signals to excite theantenna. In an exemplary embodiment, at least one input port is used tofeed the antenna with a radio frequency signal as the excitation signaland all other input ports are closed on a matched load. Also, two inputports may be connected to an RF switch that alternatively allowsexciting one or the other of the two input ports.

In a first configuration of the reconfigurable antenna of the invention,the CRLH unit cells are cascaded along a straight line and the DC biasused to change the variable capacitance and/or inductance in parallelwith the series capacitance is used to control the radiation angle whilethe DC bias used to change the variable capacitance and/or inductance inseries with the shunt inductance is used to control the radiation angle,the polarization of the radiated electrical field, and impedancematching.

In a second configuration of the reconfigurable antenna of theinvention, the CRLH unit cells are cascaded with a zigzag shape wherebyrespective CRLH unit cells are substantially orthogonal to each otherand the DC bias used to change the variable capacitance and/orinductance in parallel with the series capacitance is used to controlthe radiation angle while the DC bias used to change the variablecapacitance and/or inductance in series with the shunt inductance isused to control the radiation angle, the polarization of the radiatedelectrical field, and impedance matching. Preferably, the CRLH unitcells are interleaved with a variable phase shifter that dynamicallycontrols the polarization of the radiated electrical field. Also, acapacitor may be used in an exemplary configuration to decouplerespective DC bias networks that generate the two DC biases.

In a third configuration of the reconfigurable antenna of the invention,the CRLH unit cells are cascaded along a circular arc and the DC biasused to change the variable capacitance and/or inductance in parallelwith the series capacitance is used to control the polarization of theradiated field while the DC bias used to change the variable capacitanceand/or inductance in series with the shunt inductance is used to controlthe polarization of the radiated field and impedance matching. In anexemplary configuration, pairs of the CRLH unit cells are displacedorthogonally in space along the circular arc. A capacitor may also beincluded in the circuit to decouple respective DC bias networks thatgenerate the at least two DC biases.

The invention also includes methods of varying pattern and/orpolarization of a reconfigurable antenna by providing at least oneComposite Right Left Handed (CRLH) unit cell including a standardtransmission line with added series capacitance and shunt inductance andadapted to radiate an electrical field and at least a variablecapacitance and/or inductance in series with the shunt inductance and atleast a variable capacitance and/or inductance in parallel with theseries capacitance and separately applying at least two DC biases to thevariable capacitance and/or inductance in series with the shuntinductance and the variable capacitance and/or inductance in parallelwith the series capacitance to independently control the variablecapacitance and/or inductance in parallel with the series capacitanceand the variable capacitance and/or inductance in series with the shuntinductance so as to thereby control the group delay of the transmissionline and a polarization of the radiated electrical field. Multiple CRLHunit cells are cascaded to define a leaky wave structure, and excitationsignals are applied to at least one input port of the leaky wavestructure to excite the antenna. At least one input port is fed with aradio frequency signal as the excitation signal while all other inputports are closed on a matched load. Also, two input ports may bealternately excited by selectively opening and closing an RF switchbetween the two input ports.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. Furthermore,the claimed subject matter is not limited to implementations that solveany or all disadvantages noted in any part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will be described in connectionwith the associated figures, of which:

FIG. 1 illustrates a Composite Right Left Handed (CRLH) transmissionline unit cell schematic (FIG. 1( a)) and equivalent circuit model (FIG.1( b)).

FIG. 2 illustrates a dispersion diagram of a CRLH transmission line unitcell.

FIG. 3 illustrates a reconfigurable CRLH transmission line unit cellschematic (FIG. 3( a)) and dispersion diagram (FIG. 3( b)).

FIG. 4 illustrates a CRLH tunable unit cell with independent biasingnetworks and good impedance matching schematic (FIG. 4( a)) and circuitmodel (FIG. 4( b)) in accordance with the invention.

FIG. 5 illustrates a dispersion diagram of the unit cell of theinvention for four different bias voltage combinations.

FIG. 6 illustrates a two port reconfigurable leaky wave antenna (LWA)for use in accordance with the invention.

FIG. 7 illustrates measured scattering parameters for four differentconfigurations of the reconfigurable LWA used in accordance with theinvention.

FIG. 8 illustrates measured radiation patterns excited at the two portsof the reconfigurable LWA for four different configurations at port 1(FIG. 8( a)) and at port 2 (FIG. 8( b)) at a frequency of 2.44 GHz.

FIG. 9 illustrates measured radiation patterns excited at port 1 of thereconfigurable LWA for four different configurations for verticalpolarization (FIG. 9( a)) and horizontal polarization (FIG. 9( b)) at afrequency of 2.44 GHz.

FIG. 10 illustrates a schematic of the polarization reconfigurable LWAof the invention where pairs of cells with the same number areorthogonal in space.

FIG. 11 illustrates an embodiment of the LWA of FIG. 10 with frequencydependent polarization reconfigurability.

FIG. 12 illustrates axial ratio as function of the propagation constantβ for a CRLH cell configuration where the linear polarization condition(β=0 rad/m) is obtained at the frequency of 840 MHz.

FIG. 13 illustrates radiation patterns for different frequency ofoperations where the mean beam direction is independent from thepolarization/propagation constant for (a) φ=0° and (b) φ=90°.

FIG. 14 illustrates a CRLH reconfigurable unit cell schematic (FIG. 14(a)) and a dispersion diagram (FIG. 14( b)) for different values ofapplied voltages “S” and “SH”.

FIG. 15 illustrates an embodiment of the LWA with frequency dependentpolarization reconfigurability.

FIG. 16 illustrates radiation patterns for four different configurationsof the LWA with frequency independent polarization reconfigurability for(a) φ=0° and (b) φ=90° at a frequency of 880 MHz.

FIG. 17 illustrates a schematic of a pattern and polarizationreconfigurable CRLH LWA in accordance with the invention.

FIG. 18 illustrates an embodiment of a polarization reconfigurable LWAwith frequency dependent beam scanning capabilities.

FIG. 19 illustrates axial ratio for three different angles of radiationat the frequencies of 800 MHz, 865 MHz and 970 MHz for different valuesof phase shift (PS1=−PS2).

FIG. 20 illustrates radiation patterns for different frequencies ofoperation illustrating that under the condition PS1=−PS2 the beamdirection is independent from the applied phase shift.

FIG. 21 illustrates an embodiment of the polarization reconfigurable LWAwith frequency independent beam scanning capabilities.

FIG. 22 illustrates radiation patterns for four different configurationsof the reconfigurable LWA with frequency independent beam scanningcapabilities for (a) 2p=d and (b) 2p/d=1.2 for a frequency of 880 MHz.

FIG. 23 illustrates axial ratio in the direction of maximum radiationfor four different configurations of the pattern and polarizationreconfigurable LWA in function of phase shift values (PS1=−PS2) at afrequency of 880 MHz.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A detailed description of illustrative embodiments of the presentinvention will be described below with reference to FIGS. 1-23. Althoughthis description provides detailed examples of possible implementationsof the present invention, it should be noted that these details areintended to be exemplary and in no way delimit the scope of theinvention.

A leaky wave is a traveling wave that progressively leaks out powerwhile it propagates along a waveguiding structure. Such structures areusually used as antennas to achieve high directivity. Leaky waveantennas are fundamentally different from resonating antennas in thesense that they are based on a traveling wave as opposed to a resonatingwave mechanism. Significantly, the antenna size is not related to theantenna resonant frequency but to its directivity.

The radiation properties of a leaky wave antenna are related to thepropagation constant along the direction of the waveguide, γ=α−jβ (whereα is the attenuation constant and β is the phase constant), and to thepropagation constant perpendicular to this direction, k⊥. The twopropagation constants are related as:

k _(⊥)=√{square root over (k ₀ ²−β²)}

where k₀ is the free space wave number.

If the wave is slower than the velocity of light (slow wave region) andso k₀<β, the perpendicular propagation constant, k⊥, is imaginary andtherefore no radiation occurs, and the wave is guided. If, in contrast,the wave is faster than the velocity of light (fast wave region) and sok₀>β, the perpendicular propagation constant is real and radiationoccurs. In particular, radiation occurs under the angle

$\theta = {\sin^{- 1}\left( \frac{\beta}{k_{0}} \right)}$

where θ is the maximum beam angle from the broadside direction. Thus,the radiation angle can be controlled by frequency in a leaky waveantenna. The attenuation constant, α, determines instead the radiatedpower density per unit length. For large values of α most of the poweris leaked in the first part of the waveguiding structure, while forsmall values of α, leakage occurs slowly and highly directivity isachieved.

A dominant mode frequency-scanned LW antenna can be implemented usingcomposite right left handed (CRLH) transmission lines. A CRLHtransmission line is implemented by inserting an artificial seriescapacitance and a shunt inductance into a conventional transmission linewhich has an intrinsic series inductance and shunt capacitance. Thegeneral representation of the CRLH transmission line and its equivalentcircuit model are shown in FIG. 1. As illustrated, the CRLH transmissionline includes an interdigital capacitor and a shorted shunt stubrepresenting a series capacitance and a shunt inductance, respectively.

Loading a common transmission line with a series capacitance and shuntinductance allows for the creation of a metamaterial that modifies thetypical propagation characteristic of right handed (RH) materials whichare characterized by a positive propagation constant, β>0. In CRLHtransmission lines the material propagation behavior shifts withfrequency from RH (characterized by β>0) to left handed (LH)(characterized by β<0). This effect has been demonstrated by Caloz etal. in “Transmission line approach of left-handed (LH) materials andmicrostrip implementation of an artificial LH transmission line,” IEEETransactions on Antennas and Propagation, Vol. 52, No. 5, pp. 1159-1166(2004) and by Lai et al. in “Composite right/left-handed transmissionline meta-materials,” IEEE Microwave Magazine, Vol. 5, No. 3, pp. 34-50(2004) and it can be observed in the dispersion diagram of FIG. 2.According to the CRLH transmission line dispersion diagram of FIG. 2,there are four distinct regions: the LH-guided region, the LH-leakyregion, the RH-leaky region and the RH-guided region. This backfire toendfire scanning capability, first demonstrated experimentally by Sanadaet al. in “Characteristics of the composite right/left-handedtransmission lines,” IEEE Microwave and Wireless Components Letters,Vol. 14, No. 2, pp. 68-70 (2004) and explained by the CRLH concept byCaloz et al. in “A novel composite right/left-handed coupled-linedirectional coupler with arbitrary coupling level and broad bandwidth,”IEEE Transactions on Microwave Theory and Techniques, Vol. 52, No. 3,pp. 980-992 (2004), is a very unique feature for a LWAs, which cannot beobtained in conventional leaky wave structures.

The frequency scanned nature of these LWA is, however, a disadvantagethat has limited their applications in modern communication systems,generally requiring fixed frequency operation for effectivechannelizing. In CRLH LWA, since the main radiation beam angle is afunction of the propagation constant along the structure, it is possibleto steer the beam by LC parameters tuning at a fixed frequency ofoperation. In this case, varactor diodes can be integrated along thestructure, in each cell, to provide continuously variable capacitancesor variable inductance via the control of their reverse bias voltage V.A first prototype of an electronically scanned CRLH LWA has beenproposed by Sungjoon et al. in “Metamaterial-based electronicallycontrolled transmission-line structure as a novel leaky-wave antennawith tunable radiation angle and beamwidth,” IEEE Transactions onMicrowave Theory and Techniques, Vol. 52, December 2004, and its workingprinciple is described in the dispersion diagram of FIG. 3( b) for thereconfigurable CRLH transmission line unit cell generally illustrated inFIG. 3( a). As illustrated, by varying the applied bias voltage V, it ispossible to shift the propagation characteristics of the transmissionline and achieve different propagation constants, β, for a fixedfrequency of operation.

The unit cell structure presented by Sungjoon et al. has beendemonstrated to be effective for building LWAs that allow changing thedirection in which the beam is steered. However, LWAs built using thistype of unit cell suffer from a gain imbalance between the differentconfigurations. The design of CRLH unit cells presented by Sungjoon etal. is conceived to have l<<λ_(g), with λ_(g) being the guidedwavelength and l the unit cell length, and to have variable capacitancecontrolled simultaneously through a single DC bias. Using this design,several unit cells need to be used in order to achieve gooddirectionality, and this causes the antenna to have low gain forconfigurations that do not point in broadside as well as insufficientimpedance matching. Also, to date the properties of CRLH materials havebeen used to build LWAs capable only of steering the beam continuouslyfrom end-fire to back-fire.

The invention relates to a novel structure of CRLH unit cell that allowsfor exploitation of the characteristic behavior of CRLH to build LWAscapable of simultaneously changing pattern and polarization whilepreserving good impedance matching and high gain for all the antenna'sconfigurations. An exemplary embodiment of an exemplary embodiment ofthe metamaterial unit cell structure of the invention is shown in FIGS.4( a) and 4(b). In order to achieve CRLH behavior, the unit cell of FIG.4 is designed by inserting an artificial series capacitance and a shuntinductance into a conventional microstrip line by means of aninterdigital capacitor and a shorted stub respectively. To dynamicallytune the handedness of the unit cell, two varactor diodes (D_(S)) areplaced in parallel with the microstrip series interdigital capacitor andone varactor diode (D_(SH)) is placed in series with the shunt inductor.Two independent bias networks are used to separately tune the varactorsD_(S) (“S” bias) and D_(SH) (“SH” bias). A capacitor (C=0.5 pF) is usedto decouple the two DC bias networks, and quarter wave transformers areemployed to prevent the RF signal from flowing to DC ground. By usingtwo independent DC bias networks, it is possible to adjust the unit cellreactance in order to keep the Bloch impedance close to 50Ω whileshifting the unit cell electrical characteristic from left hand to righthand. Moreover, the use of a separate D_(SH) (“SH” bias) bias networkallows changing the unit cell polarization. This property caneffectively be used to also control the polarization in CRLH LWAs.

The CRLH unit cell, differently from any proposed approach, needs tohave l˜λ_(g)/4 while preserving the characteristic CRLH behavior. Usingunit cells with size comparable to λ_(g)/4 allows building high gainLWAs composed of few unit cells with overall low losses introduced bythe active components. This technique allows building active LWAs withstrong gain.

Three exemplary embodiments of pattern and polarization reconfigurableantennas have been designed using this type of CRLH unit cell structure.The working principle of these antennas is unique and is part of thisinvention.

Antenna Design 1

A leaky wave antenna (LWA) in accordance with antenna design 1 usescomposite right left handed (CRLH) materials in order to achieve highradiation pattern and polarization reconfigurability without sacrificinggain, impedance matching, or compactness. Two separate ports are locatedon the same antenna structure so that a single physical antenna can beused as a two elements array for reduced antenna space occupation on thecommunication device. The leaky wave antenna is composed of N cascadedCRLH unit cells. An embodiment of this unit cell is built on Rogerssubstrate with a length, l, of 13 mm. Skyworks SMV1413 varactor diodeswith a measured capacitance that varies continuously from 1.3 pF (for abias voltage of 40 Volts) to 7.3 pF (for a bias voltage of 0 Volt) areused.

FIG. 5 shows the measured dispersion diagram of the proposed unit cellfor four different configurations of “S” and “SH” DC bias voltages.Table I shows the measured Bloch impedance for the same voltagecombinations at a frequency of 2.44 GHz. It will be appreciated thatthis unit cell design allows for continuous shifting of the propagationconstant, β, for a fixed frequency of operation while keeping the Blochimpedance close to 50Ω. This unit cell design is then suitable forbuilding reconfigurable CRLH LWAs with good matching over the entire setof generated scanning beams. For a selected frequency of operation, inthe fast wave region of the unit cell, β<k₀, radiation occurs at theangle:

$\theta = {\sin^{- 1}\left( \frac{\beta}{k_{0}} \right)}$

where θ is the radiation angle and k₀ is the free-space wavenumber.

TABLE I CONFIGURATION IMPEDANCE [Ω] S = 9 V SH = 7 V 45 + j5 S = 30 V SH= 20 V 65 + j7 S = 18 V SH = 15 V 40 + j10 S = 10 V SH = 8 V 43 + j4

FIG. 6 shows a prototype of a two port reconfigurable leaky wave antennabuilt with the unit cell structure having the dispersion diagramillustrated in FIG. 5. The antenna includesg 10 unit cells and has beendesigned to operate at the frequency of 2.44 GHz. The design is 14 cmlong and it allows for excitation of two independent beams (one perport) that can be steered from backfire to endfire. Since a commonantenna structure is used for the two ports, the excited beams aresteered together symmetrically with respect to the, broadside direction.Ideally, since the varactor capacitance allows for continuous tuning, aninfinite number of configurations can be selected for the antenna.

FIG. 7 shows the measured scattering parameters for four different arrayconfigurations (each corresponding to a specific combination of “S” and“SH” voltages). Both ports are matched at the frequency of 2.44 GHz withrespect to a 10 dB target return loss. The isolation between the twoports is higher than 10 dB for all the configurations.

FIG. 8 shows the measured radiation patterns excited at port 1 (FIG. 8(a)) and at port 2 (FIG. 8( b)) at a frequency of 2.44 GHz for the samefour different array configurations of FIG. 7. As illustrated, the beamcan be effectively steered over 90° in the elevation plane with minordifferences between the two ports. The beam scanning direction of theproposed antenna structure can be predicted using the dispersion diagraminformation as:

$\theta_{1} = {{\sin^{- 1}\left( \frac{\beta \left( {S,{SH}} \right)}{k_{0}} \right)} = {- \theta_{2}}}$

where θ₁ and θ₂ are the scanning angles at port 1 and port 2. Assummarized in Table II, the antenna measured scanning direction agreeswell with the one predicted using the measured propagation constant of asingle unit cell.

FIG. 9 shows the measured radiation patterns for the verticalpolarization (FIG. 9( a)) and horizontal polarization (FIG. 9( b)) at afrequency of 2.44 GHz excited at one port of the LWA. It can be notedthat using an independent DC bias (“SH”) to change the values of shuntcapacitance, the polarization of the antenna can be effectively changedfor a given pointing direction. The antenna can then also be used tochange the polarization of the radiated beam while changing its pointingdirection.

TABLE II 2.4 GHz 2.44 GHz 2.48 GHz Est. Est. Est. S SH Angle Measured RLAngle Measured RL Angle Measured RL [V] [V] [deg] [deg] [dB] [deg] [deg][dB] [deg] [deg] [dB] 30 20 −42 −60 16 −17 −40 12 −4 −10 15 10 8 30 2014 48 25 17 68 40 17 18 15 −16.5 −25 12 −2.5 −10 12 0.7 −5 5.3 9 7 53 3011 83 35 15 >90 45 20

Antenna Design 2

In this embodiment, the properties of CRLH materials are exploited toachieve polarization tunability in leaky wave antennas with broadsideradiation.

A LWA antenna with variable polarization can be designed by cascading NCRLH unit cells with linear polarization along a semi-circumference asshown in FIG. 10. The N cells are arranged in that shape to achievevariable polarization depending on the value of the unit cellpropagation constant β, and a frequency/polarization independentbroadside radiation pattern. Pairs of cells are displaced orthogonallyin space along the semi-circumference, as shown in FIG. 10, to obtaintwo orthogonal electric field components.

The difference in phase excitation between each cell that constitutes apair (e.g. the orthogonal cells marked as 1 in FIG. 10) is a function ofthe unit cell propagation constant and it determines the polarization ofthe radiated field. A phase difference of 0° between two orthogonalcells is achieved for β=0°, and the LWA radiates in broadside withlinear polarization (LP). In the left hand region (β<0°) the antennaradiates with right hand (RH) polarization while in the right handregion (β>0°) it radiates with left hand (LH) polarization. The phasedifference, Δφ, of the excitation of two orthogonal unit cells is givenby:

Δφ=−(K+1)βp

where K is the number of CRLH unit cells that separates the twoorthogonal cells. The difference in amplitude, Δl, between theexcitation of two orthogonal unit cells is defined as:

ΔI=I ₀(1−e ^(−(K+1)αp))

where I₀ is the current at the input port of the LWA and a is theattenuation constant of the CRLH TL. Since two orthogonal unit cellscannot be excited with equal magnitude, pure circular polarizationcannot be generated.

An exemplary embodiment of this antenna structure is a LWA withfrequency dependent polarization reconfigurability. The design of theCRLH unit cell for this embodiment is shown in FIG. 11. To achieve thedesired CRLH behaviour, the unit cell is designed using an interdigitalcapacitor and a shunt lumped inductor. A lumped inductor is used insteadof a longer shorted stub to design a unit cell with strong linearpolarization.

As illustrated in FIG. 11, N=12 unit cells are cascaded along asemi-circumference. The antenna, built on a Rogers 4003C substrate, isfed at one port while the other port is closed on a matched load. Themain structural parameters of the antenna are shown in Table III.

TABLE III STRUCTURAL PARAMETERS OF THE LWA WITH FREQUENCY DEPENDENT BEAMSCANNING CAPABILITIES p 31.6 mm l_(s) 4.5 mm L 6 nH h 3.3 mm ε_(r) 3.55r 12.9 cm

FIG. 12 illustrates the LWA axial ratio as a function of the unit cellpropagation constant, β, in the broadside direction. The antennapolarization can be continuously changed from right hand circularpolarization (RHCP) to left hand circular polarization (LHCP) by varyingthe frequency of operation. The axial ratio can be tuned to 1 dB (LHCP)at the frequency of 930 MHz (βp=0.25 rad that corresponds to a phasedifference of −90° between each pair of orthogonal cells), 40 dB (LP) atthe frequency of 860 MHz (βp=0 rad that corresponds to a phasedifference of 0° between each pair of orthogonal cells) and 6 dB (RHelliptical polarization) at the frequency of 790 MHz (MHz (βp=−0.25 radthat corresponds to a phase difference of 90° between each pair oforthogonal cells). An imbalance between the axial ratios of the RH andLH regions is due to the asymmetric structure of the unit cell.

The semi-circular shape allows also for broadside radiationindependently from the frequency of operation. FIG. 13 shows thesimulated radiation patterns of the antenna for different frequencies ofoperation where the mean beam direction is independent from thepolarization/propagation constant for (a) φ=0° and (b) φ=90°. Asillustrated, the antenna gain is constant independently from theradiated polarization and it falls in the range [0, +1] dBi. The returnloss is less than 10 dB in the UHF band (790 MHz-930 MHz).

Another exemplary embodiment of this antenna structure is a LWA withfrequency independent polarization reconfigurability. Loading the CRLHunit cell with varactor diodes, the propagation characteristics of theCRLH transmission line (TL) can be varied for a given frequency ofoperation.

The modified CRLH unit cell is shown in FIG. 14( a). As illustrated, twovaractor diodes, D_(S), are placed in parallel with the microstripseries interdigital capacitor IC and one varactor diode D_(SH) is placedin series with the shunt inductor L. Two independent bias networks areused to separately tune the varactors D_(S) (“S” voltage) and D_(SH)(“SH” voltage). A capacitor C (C=0.5 pF) is used to decouple the two DCbias networks. The CRLH unit cell is built on Rogers 4003 substrate andthe scattering parameters of Skyworks SMV1413 varactor diodes have beenused together with simulations based on the method of moments todetermine the electrical properties of the CRLH unit cell. Thecapacitance of the selected varactor diodes can be tuned from 10.1 pF to1.6 pF to vary the applied voltage from 0V to 30V at the frequency of880 MHz. The simulated dispersion diagrams of the reconfigurable CRLH ofFIG. 14( a) are shown in FIG. 14( b) for different values of appliedvoltages “S” and “SH”. It will be appreciated that the propagationconstant, β, varies with the applied DC bias for the same frequency ofoperation.

As shown in FIG. 15, N=10 cells are cascaded along a semi-circumferenceto obtain a polarization reconfigurable LWA. The LWA is capable ofchanging the polarization state of the radiated field by properly tuningthe applied voltages “S” and “SH” while radiating in broadside. FIG. 16shows the simulated radiation patterns of the antenna with frequencyindependent polarization reconfigurability for different configurationsof applied voltages for (a) φ=0° and (b) φ=90° at a frequency of 880MHz. Table IV reports the axial ratios and the gains of four differentconfigurations. The antenna is capable of changing the polarization ofthe radiated field from linear (configuration “SH=20V−S=5V”) to circular(RHCP for configuration “SH=30V−S=10V”, LHCP for configuration“SH=15V−S=2V”). However, the structure suffers from low gain that can beincreased by using more unit cells displaced along a semi-circumferenceof longer radius.

TABLE IV AXIAL RATIO AND GAIN FOR DIFFERENT CONFIGURATIONS OF THERECONFIGURABLE LWA. Configuration AR [dB] Gain [dBi] S = 10 V - SH = 0 V14.2 0.8 S = 15 V - SH = 2 V 2.2 1.1 S = 20 V - SH = 5 V 21.4 2.8 S = 30V - SH = 10 V 2.7 0.5 FREQUENCY = 880 MHZ

Antenna Design 3

The antenna design of this embodiment includes a reconfigurable leakywave antenna (LWA) that takes advantage of the CRLH properties toachieve full pattern and polarization reconfigurability.

In this embodiment, two consecutive CRLH unit cells characterized bylinear polarization are displaced orthogonally, in V shape, as shown inFIG. 17, to radiate two orthogonal electric fields. A variable phaseshifter (PS1) placed across two consecutive unit cells allows control ofthe phase difference between the two arms of the V structure. Byproperly adjusting the phase shift from −90° to +90°, the polarizationof the V structure can be changed (in the broadside direction) fromright hand to left hand circular. Linear polarization is achieved for aphase shift of 0°. In the embodiment of FIG. 17, a pattern andpolarization reconfigurable LWA is obtained by cascading N V cellsinterleaved with a variable phase shifter, PS2, used to compensate thephase shift introduced by PS1.

This zigzag LWA of FIG. 17 is equivalent to an array of non directiveradiating elements with variable polarization (V cells) andinter-element spacing d. The phase excitation, ξ_(n), of the n-the arrayelement is:

ξ_(n)=−(n−1)2βp

and the current excitation, I_(n), is

I _(n) =I ₀ e ^(−(n−)1)2αp

where I₀ is the current at the input port of the LWA and a is theattenuation constant of the CRLH TL. The maximum radiation angle, θ, ofsuch LWA can be predicted as:

$\theta = {{\sin^{- 1}\left( \frac{2\beta \; p}{k_{0}d} \right)}.}$

The beam direction of this LWA is controlled through the TL propagationconstant, β, while the polarization of the radiated field can bedynamically varied through the phase shifters, PS1 and PS2. In thisdesign, unlike in conventional CRLH LWAs, it is possible to achieveend-fire radiation for values of 0<β<1 and back-fire radiation forvalues of −1<β<0 by properly setting the ratio 2p/d.

An exemplary embodiment of this antenna structure is a LWA withfrequency dependent pattern reconfigurability. The design of the CRLHunit cell for this preferred embodiment is shown in FIG. 18. FIG. 18shows a prototype of this antenna designed using N=8 V cells on a Rogers4003C substrate. The antenna is fed at one port while the other port isclosed on a matched load. The polarization of the LWA can be changedcontinuously from circular to linear in the broadside direction bytuning PS1 to control the polarization of each V cell and using PS2 tocompensate for the phase shift of PS1 (PS1=−PS2). Right hand circularpolarization is achieved for PS1=90° and PS2=−90°. The values of axialratios for the simulated broadside radiation patterns at the frequenciesof 800 MHz, 865 MHz and 970 MHz for different values of phase shift(PS1=−PS2) are shown in FIG. 19 for different values of phase shift.

FIG. 20 illustrates the antenna radiation patterns for differentfrequencies of operations simulated using the Method of Moments (MoM).It will be appreciated that the beam scanning capability typical of CRLHLWAs is maintained and it is a function of the dispersion curve of thesingle unit cell. Broadside radiation is observed at the frequency of865 MHz (propagation constant, β=0°), and in the left hand region (β<0°)the antenna radiates backfire and in the right hand region (β>0°) itradiates endfire. This behavior is satisfied for PS1=−PS2. Inparticular, in this design 2p/d=1 and therefore the radiation angle, θ,is defined as:

$\theta = {{\sin^{- 1}\left( \frac{\beta}{k_{0}} \right)}.}$

Another exemplary embodiment of this antenna structure is a LWA withfrequency independent polarization reconfigurability. Loading the CRLHunit cell with varactor diodes, the propagation characteristics of theCRLH TL can be varied for a given frequency of operation.

The modified CRLH unit cell of FIG. 14( a) may be used in thisconfiguration. As described above, two varactor diodes, D_(S), areplaced in parallel with the microstrip series interdigital capacitor ICand one varactor diode D_(SH) is placed in series with the shuntinductor L. Two independent bias networks are used to separately tunethe varactors D_(S) (“S” voltage) and D_(SH) (“SH” voltage). A capacitorC (C=0.5 pF) is used to decouple the two DC bias networks. The CRLH unitcell is built on Rogers 4003 substrate and the scattering parameters ofSkyworks SMV1413 varactor diodes are used together with simulationsbased on the MoM to determine the electrical properties of the CRLH unitcell. The capacitance of the selected varactor diodes can be tuned from10.1 pF to 1.6 pF to vary the applied voltage from 0V to 30V at thefrequency of 880 MHz. The simulated dispersion diagrams of thereconfigurable CRLH are shown in FIG. 14( b) for different values ofapplied voltages “S” and “SH”. It will be appreciated that thepropagation constant, β, varies with the applied DC bias for the samefrequency of operation.

In the embodiment of FIG. 21, N=8 V cells are cascaded to obtain apattern and polarization reconfigurable LWA. The antenna of FIG. 21 iscapable of changing the direction of radiation for a fixed frequency ofoperation by properly tuning the applied voltages “S” and “SH”. Theradiation angle, θ, is defined as

$\theta = {{\sin^{- 1}\left( \frac{2{\beta \left( {S,{SH}} \right)}p}{k_{0}d} \right)}.}$

FIG. 22( a) shows the simulated radiation patterns for a discrete set ofapplied voltages at the frequency of 865 MHz with 2p=d. As illustrated,configuration “SH=20V−S=5V” has the maximum gain (4.5 dBi), whileconfiguration “SH=10V−S=0V” exhibits the minimum gain (0.5 dBi). Byproperly tuning the phase shifters PS1 and PS2, the polarization of theradiated field can be varied in the direction of maximum radiation. Theaxial ratio in the direction of maximum radiation is shown in FIG. 23for different values of applied voltages and phase shifts.

In addition, by properly selecting the ratio 2p/d it is possible toachieve full scanning from backfire to endfire independently from therange of tunability of the variable capacitors. FIG. 22( b) shows thesimulated radiation patterns of a LWA where 2p/d=1.2. It will beappreciated that for the same values of applied voltages the antennascanning range is increased 27° with respect to the LWA design where2p=d (see FIG. 22( a)).

In the antenna designs in accordance with the invention, the antenna'sproperties are reconfigured by means of variable capacitors. It is alsonoted that variable inductors can be used to achieve a similar behavior.It is also noted that in the described embodiments only one port isactivated at a time. However, it will be appreciated that the antennasystem of the invention can be used with simultaneous excitation of thetwo ports to achieve a symmetrical behavior with respect to thebroadside direction. Another technique for efficiently using the twoports of the antenna system of the invention is to employ a switch toselect the port used to feed the antenna depending on the specificwireless channel.

While the invention has been described with reference to specificembodiments, the description is illustrative of the invention and is notto be construed as limiting the invention. Various modification andapplications may occur to those skilled in the art without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

Therefore, it must be understood that the illustrated embodiment hasbeen set forth only for the purposes of example and that it should notbe taken as limiting the invention as defined by the following claims.For example, notwithstanding the fact that the elements of a claim areset forth below in a certain combination, it must be expresslyunderstood that the invention includes other combinations of fewer, moreor different elements, which are disclosed in above even when notinitially claimed in such combinations. A teaching that two elements arecombined in a claimed combination is further to be understood as alsoallowing for a claimed combination in which the two elements are notcombined with each other, but may be used alone or combined in othercombinations. The excision of any disclosed element of the invention isexplicitly contemplated as within the scope of the invention.

The words used in this specification to describe the invention and itsvarious embodiments are to be understood not only in the sense of theircommonly defined meanings, but to include by special definition in thisspecification structure, material or acts beyond the scope of thecommonly defined meanings. Thus if an element can be understood in thecontext of this specification as including more than one meaning, thenits use in a claim must be understood as being generic to all possiblemeanings supported by the specification and by the word itself.

The definitions of the words or elements of the following claims are,therefore, defined in this specification to include not only thecombination of elements which are literally set forth, but allequivalent structure, material or acts for performing substantially thesame function in substantially the same way to obtain substantially thesame result. In this sense it is therefore contemplated that anequivalent substitution of two or more elements may be made for any oneof the elements in the claims below or that a single element may besubstituted for two or more elements in a claim. Although elements maybe described above as acting in certain combinations and even initiallyclaimed as such, it is to be expressly understood that one or moreelements from a claimed combination can in some cases be excised fromthe combination and that the claimed combination may be directed to asubcombination or variation of a subcombination.

Insubstantial changes from the claimed subject matter as viewed by aperson with ordinary skill in the art, now known or later devised, areexpressly contemplated as being equivalently within the scope of theclaims. Therefore, obvious substitutions now or later known to one withordinary skill in the art are defined to be within the scope of thedefined elements.

The claims are thus to be understood to include what is specificallyillustrated and described above, what is conceptually equivalent, whatcan be obviously substituted and also what essentially incorporates theessential idea of the invention.

1. A pattern and/or polarization reconfigurable antenna comprising: atleast one Composite Right Left Handed (CRLH) unit cell including astandard transmission line with added series capacitance and shuntinductance and adapted to radiate an electrical field; and at least avariable capacitance and/or inductance in series with the shuntinductance and at least a variable capacitance and/or inductance inparallel with the series capacitance, whereby said at least a variablecapacitance and/or inductance in series with the shunt inductance andsaid at least a variable capacitance and/or inductance in parallel withthe series capacitance are responsive to at least two DC biases used toindependently control the variable capacitance and/or inductance inparallel with the series capacitance and the variable capacitance and/orinductance in series with the shunt inductance to thereby control thegroup delay of the transmission line and/or a polarization of theradiated electrical field.
 2. The reconfigurable antenna of claim 1,wherein the CRLH unit cell and at least a variable capacitance and/orinductance in series with the shunt inductance and at least a variablecapacitance and/or inductance in parallel with the series capacitanceare fabricated on a microwave laminate printed circuit board.
 3. Thereconfigurable antenna of claim 1 wherein multiple CRLH unit cells arecascaded to define a leaky wave structure.
 4. The reconfigurable antennaof claim 3 wherein the DC biases are used to control the shape and/ordirection of the radiated field, and/or the polarization of the radiatedfield, and/or the antenna input impedance.
 5. The reconfigurable antennaof claim 4 further comprising at least two input ports for acceptingexcitation signals to excite the antenna.
 6. The reconfigurable antennaof claim 5, wherein at least one input port is used to feed the antennawith a radio frequency signal as said excitation signal and all otherinput ports are closed on a matched load.
 7. The reconfigurable antennaof claim 5 wherein two input ports are connected to an RF switch thatalternatively allows exciting one input port of said two input ports orthe other input port of the two input ports.
 8. The reconfigurableantenna of claim 4, wherein the CRLH unit cells are cascaded along astraight line and the DC bias used to change the variable capacitanceand/or inductance in parallel with the series capacitance is used tocontrol the radiation angle while the DC bias used to change thevariable capacitance and/or inductance in series with the shuntinductance is used to control the radiation angle, the polarization ofthe radiated electrical field, and impedance matching.
 9. Thereconfigurable antenna of claim 4, wherein the CRLH unit cells arecascaded with a zigzag shape whereby respective CRLH unit cells aresubstantially orthogonal to each other and the DC bias used to changethe variable capacitance and/or inductance in parallel with the seriescapacitance is used to control the radiation angle while the DC biasused to change the variable capacitance and/or inductance in series withthe shunt inductance is used to control the radiation angle, thepolarization of the radiated electrical field, and impedance matching.10. The reconfigurable antenna of claim 9, wherein the CRLH unit cellsare interleaved with a variable phase shifter that dynamically controlsthe polarization of the radiated electrical field.
 11. Thereconfigurable antenna of claim 9, further comprising a capacitor thatdecouples respective DC bias networks that generate said at least two DCbiases.
 12. The reconfigurable antenna of claim 4, wherein the CRLH unitcells are cascaded along a circular arc and the DC bias used to changethe variable capacitance and/or inductance in parallel with the seriescapacitance is used to control the polarization of the radiated fieldwhile the DC bias used to change the variable capacitance and/orinductance in series with the shunt inductance is used to control thepolarization of the radiated field and impedance matching.
 13. Thereconfigurable antenna of claim 12, wherein pairs of said CRLH unitcells are displaced orthogonally in space along said circular arc. 14.The reconfigurable antenna of claim 12, further comprising a capacitorthat decouples respective DC bias networks that generate said at leasttwo DC biases.
 15. A method of varying pattern and/or polarization of areconfigurable antenna, comprising the steps of: providing at least oneComposite Right Left Handed (CRLH) unit cell including a standardtransmission line with added series capacitance and shunt inductance andadapted to radiate an electrical field and at least a variablecapacitance and/or inductance in series with the shunt inductance and atleast a variable capacitance and/or inductance in parallel with theseries capacitance; and separately applying at least two DC biases tosaid at least a variable capacitance and/or inductance in series withthe shunt inductance and said at least a variable capacitance and/orinductance in parallel with the series capacitance to independentlycontrol the variable capacitance and/or inductance in parallel with theseries capacitance and the variable capacitance and/or inductance inseries with the shunt inductance so as to thereby control the groupdelay of the transmission line and and/or polarization of the radiatedelectrical field.
 16. The method of claim 15, further comprisingcascading multiple CRLH unit cells so as to define a leaky wavestructure.
 17. The method of claim 15, further comprising applyingexcitation signals to at least two input ports of said leaky wavestructure to excite the antenna.
 18. The method of claim 17, furthercomprising feeding said at least one input port with a radio frequencysignal as said excitation signal and closing all other input ports on amatched load.
 19. The method of claim 17, further comprisingalternatively exciting two input ports by selectively opening andclosing an RF switch between said two input ports.